Polymorphism in mercury(I) selenite(IV): Preparation, crystal structures of α-, β-and γ-Hg2SeO3, and thermal behavior of the α- and β-modification

Article abstract of DOI:10.1016/S0022-4596(02)00082-8

Mercury(I) selenite(IV) is polymorphic and crystallizes at least in three modifications, named α-, β-and γ-Hg₂SeO₃. Polycrystalline β-Hg₂SeO₃ was prepared by precipitation of a concentrated mercurous nitrate solution with selenous acid. Hydrothermal treatment of the colorless β-Hg₂SeO₃ powder in demineralized water at 250°C (10days) yields light-yellow single crystals of α-Hg₂SeO₃ which show the highest density of the three modifications. Colorless needle-shaped single crystals of β-Hg₂SeO₃ and very few single crystals of γ-Hg₂SeO₃ co-crystallize from strongly diluted Hg₂(NO₃)₂ and H₂SeO₃ solutions and were grown by a diffusion technique. All crystal structures were solved and refined from single crystal diffractometer data sets and are based on Hg₂²⁺ dumbbells and trigonal pyramidal SeO₃^2- anions as the main building units. A common structural feature of all modifications is the formation of open channels extending parallel to the shortest crystallographic axis. The non-bonding orbitals of the Se^IV atoms are stereochemically active and protrude into the channels. Upon heating in an open system under N₂ atmosphere, both α- and β-Hg₂SeO₃ decompose in a well-separated three-step mechanism. The first step (T > 250°C) involves disproportionation into elementary mercury and α-HgSeO₃ which at ca. 400°C subsequently transforms into β-HgSeO₃. The second step between T = 400 and 500°C is accompanied by a loss of Hg and SeO₂ and the formation of the basic salt Hg₃SeO₆. In the third step, at temperatures between T = 500° and 600°C, this material decomposes completely. Upon heating in a closed system (sealed silica capillaries), β-Hg₂SeO₃ transforms between 320-340°C into the more dense α-Hg₂SeO₃ which on further heating likewise converts into elementary mercury and β-HgSeO₃.

Full text of DOI:10.1016/S0022-4596(02)00082-8

Journal of Solid State Chemistry 172 (2003) 35–44
Polymorphism in mercury(I) selenite(IV): preparation, crystal
structures of a-, b-and g-Hg2SeO3, and thermal behavior of the
a- and b-modification
Matthias Weil*
Institute for Chemical Technologies and Analytics, Division of Structural Chemistry, Vienna University of Technology, Getreidemarkt 9/164-SC,
A-1060 Vienna, Austria
Received 24 June 2002; received in revised form 11 September 2002; accepted 22 September 2002
Abstract
Mercury(I) selenite(IV) is polymorphic and crystallizes at least in three modifications, named a-, b-and g-Hg2SeO3. Polycrystalline
b-Hg2SeO3 was prepared by precipitation of a concentrated mercurous nitrate solution with selenous acid. Hydrothermal treatment
of the colorless b-Hg2SeO3 powder in demineralized water at 250ꢀC (10 days) yields light-yellow single crystals of a-Hg2SeO3 which
show the highest density of the three modifications. Colorless needle-shaped single crystals of b-Hg2SeO3 and very few single crystals
of g-Hg2SeO3 co-crystallize from strongly diluted Hg2(NO3)2 and H2SeO3 solutions and were grown by a diffusion technique. All
crystal structures were solved and refined from single crystal diffractometer data sets and are based on Hg22+ dumbbells and trigonal
pyramidal SeO23ꢁ anions as the main building units. A common structural feature of all modifications is the formation of open
channels extending parallel to the shortest crystallographic axis. The non-bonding orbitals of the SeIV atoms are stereochemically
active and protrude into the channels. Upon heating in an open system under N2 atmosphere, both a- and b-Hg2SeO3 decompose
in a well-separated three-step mechanism. The first step (T 4 250ꢀC) involves disproportionation into elementary mercury and
a-HgSeO3 which at ca. 400ꢀC subsequently transforms into b-HgSeO3. The second step between T = 400 and 500ꢀC is accompanied
by a loss of Hg and SeO2 and the formation of the basic salt Hg3SeO6. In the third step, at temperatures between T = 500ꢀ and
600ꢀC, this material decomposes completely. Upon heating in a closed system (sealed silica capillaries), b-Hg2SeO3 transforms
between 320-340ꢀC into the more dense a-Hg2SeO3 which on further heating likewise converts into elementary mercury and
b-HgSeO3.
r 2002 Elsevier Science (USA). All rights reserved.
Keywords: Hg2SeO3; Mercury(I); Oxoselenites(IV); Crystal structure; Polymorphism; Thermal behavior
1
2
1. Introduction
hemihydrate HgSeO3 ꢂ H2O [4], the mercury(II) selena-
te(VI) hydrate, HgSeO4 ꢂ H2O [5], and the mercury(I)
In spite of the fact that many mercury oxoselenite(IV)
and oxoselenate(VI) phases have been reported for a
very long time, the knowledge about the crystal-
lographic properties and structures of these compounds
is still deficient. For example, in the Gmelin textbook on
mercury [1] about 20 mercury salts of selenium oxoacids
with different oxidation states for both Hg and Se are
listed, but only for a few representatives complete
structural analyses have been performed in the past,
including the a-modification of mercury(II) selenite(IV),
HgSeO3 [2], the acidic Hg3(HSeO3)2(SeO3)2 [3], the
selenate(VI), Hg2SeO4 [6].
In an attempt to synthesize different phases in the
system Hg–Se–O already compiled in Ref [1], single
crystals of the following compounds were prepared and
their crystal structures analyzed: anhydrous HgSeO4,
the two basic mercury(II) selenates(VI) HgSeO4 ꢂ HgO
and HgSeO4 ꢂ 2HgO [7], the mercury(II) compounds
with mixed-valent selenium (HgSeIVO3)2 ꢂ HgSeVIO4 [8]
and (HgSeIVO3)3 ꢂ HgSeꢁII [9], and two additional
modifications of mercury(II) selenite(IV), b- and g-
HgSeO3 [9]. As part of this project the preparation and
single-crystal growth of mercury(I) selenite(IV),
Hg2SeO3, was investigated. Astonishingly, in the
most recent review on metal selenites(IV) [10], neither
*Fax: +43-1-58801-17199.
E-mail address: mweil@mail.zserv.tuwien.ac.at (M. Weil).
0022-4596/03/$ - see front matter r 2002 Elsevier Science (USA). All rights reserved.
doi:10.1016/S0022-4596(02)00082-8

36
M. Weil / Journal of Solid State Chemistry 172 (2003) 35–44
properties nor any other data for this simple salt are
reported, although preparation, appearance and solubi-
lity have presumably been described for the first time by
Berzelius nearly 200 years ago [11–13].
days, an X-ray powder diffraction (XRPD) pattern
of the polycrystalline product was measured on a
Philips X‘Pert system (CuKa1,2 radiation (l = 1.54060,
(
1.54439 A), Bragg–Brentano geometry, silicon single-
crystal sample holder, 5–120ꢀ/(2y), 0.02ꢀ/step, 10 s/step).
Indexation [14] of the so-obtained powder diagram
revealed a single-phase product and indicated a C-
centred orthorhombic cell. Refinement [15] of the lattice
(
parameters using silicon powder (a = 5.431021 A [16])
(
as an internal standard gave a = 11.1528(3) A, b =
In this article, single-crystal growth and structural
analyses of three modifications of mercury(I) selenite(IV),
a-, b- and g-Hg2SeO3, are reported, and the relations
between the crystal structures are briefly discussed. The
modification with the highest density is named a-Hg2SeO3,
and, according to the decreasing density, the other
polymorphs are denoted as b- and g-Hg2SeO3. Results
of the thermal behavior of the a- and b-modification both
in an open and a closed system are described.
(
(
16.2291(4) A and c = 5.0372(1) A. The obtained powder
pattern is in good agreement with the simulation
calculated on the basis of the subsequently refined
single-crystal data (Fig. 1).
Light-yellow single crystals of a-Hg2SeO3 with an
average diameter of ca. 0.05 mm and mostly pinacoidal
habit were obtained after hydrothermal treatment of
polycrystalline b-Hg2SeO3 in demineralized water (steel
autoclave equipped with Teflon inlays with 10 cm3
capacity, filling degree ca. 70%, 10 days, 250ꢀC). No
other phase was found by XRPD analysis of these
batches. If stoichiometric amounts of Hg2(NO3)2 ꢂ 2H2O
and SeO2 were employed as starting materials for
syntheses under similar hydrothermal conditions (demi-
neralized water with a few drops of HNO3 added to
increase the solubility and to avoid formation of basic
2. Experimental
2.1. Preparation
Precipitation of a mercurous nitrate solution, slightly
acidified with HNO3, with a selenous acid solution of
10%mol in excess (e.g. 1.123 g Hg2(NO3)2 ꢂ 2H2O
(Merck, p.A.) and 0.2440 g SeO2 (Merck, p.A.)),
produced colorless, poorly soluble polycrystalline b-
Hg2SeO3. After washing with mother liquor, ethanol
and acetone and drying in an desiccator over CaCl2 for 2
10000
5000
0
50
100
2Theta
Fig. 1. b-Hg2SeO3. The Rietveld plot [15] with comparison of observed (points) and calculated profiles (solid line) and the corresponding difference
pattern.

M. Weil / Journal of Solid State Chemistry 172 (2003) 35–44
37
salts), only few a-Hg2SeO3 was obtained and a-HgSeO3
[2] was the main phase for these batches. Dissolution of
polycrystalline b-Hg2SeO3 in a boiling diluted nitric acid
solution and subsequent recrystallization during con-
trolled cooling (10ꢀC hꢁ1) yielded also single crystals of
a-Hg2SeO3, but the crystals prepared by this method
were considerably poorer in quality than the hydro-
thermally synthesized material.
two kinds of needle-shaped crystals could not be
distinguished by optical methods, but the quality of
the measured intensity data was considerably better for
crystals with the unexpected cell than for those of the
b-modification.
Subsequent structure analyses were performed with
the SHELX97 program package [17] and the expected
formula Hg2SeO3 was confirmed for all measured
crystals. All structures were solved by determination of
the mercury atoms using direct methods and by location
of the selenium and oxygen atoms from subsequent
Fourier syntheses. Owing to the high absorption (m E
75 mmꢁ1) and the anisotropic crystal habit, an absorp-
tion correction was applied for all data sets. In case of a-
Hg2SeO3, the crystal shape was optimized by minimizing
the internal Ri value for symmetry-related reflections
using the program HABITUS [18]. The so-derived habit
was the basis for the numerical absorption correction. In
the case of the needle-shaped crystals of b- and g-
Hg2SeO3, distances of indexed faces were measured1 and
used for the analytical absorption correction as imple-
mented in the SHELXTL program [19]. I n the final
refinement cycles for all data sets, corrections of
extinction effects were applied (SHELX97) and aniso-
tropic displacement parameters allowed for all atoms in
a- and g-Hg2SeO3. All investigated b-Hg2SeO3 crystals
were racemically twinned. For the finally chosen crystal a
twin ratio of approximately 1:1 was determined (Flack
parameter [20] = 0.54(15)). No additional symmetry for
this structure was suggested by the PLATON program
[21]. Owing to the poor quality of the b-Hg2SeO3 crystals,
Se and O atoms were refined isotropically with the Ueq
parameters restrained for all oxygen atoms. Further
details of single-crystal measurement and refinement for
all modifications are summarized in Table 1, and final
atomic coordinates and anisotropic displacement para-
meters are listed in Tables 2 and 3, respectively. Selected
interatomic distances and angles, as well as the bond-
valence sums (BVS), are given in Table 4. The structure
representations were produced with the program
ATOMS [22]. Additional crystallographic information
on each compound is available from the Fachinforma-
tionszentrum Karlsruhe, D-76344 Eggenstein-Leopold-
shafen, Germany, E-mail: crysdata@fiz-karlsruhe.de, by
quoting the literature citation, the name of the author
and the depository numbers listed at the end of Table 1.
Single crystals of b-Hg2SeO3 and g-Hg2SeO3 were
grown by a diffusion technique. Two 100 ml beakers
containing diluted solutions of Hg2(NO3)2 and H2SeO3,
respectively, were placed in a larger beaker. The small
beakers were filled up to the maximum level to avoid
bubbles remaining in the solution. Then a paper filter
was placed on top of the fringes and 5 l demineralized
water were carefully charged into the larger beaker
without removing the paper filters. A few drops of
concentrated nitric acid were added to the demineralized
water to increase the solubility of the mercurous nitrate
and to prevent formation of basic salts. This device was
then placed in a dark room and set aside for 3 weeks. At
the bottom of the large beaker and on top of the paper
filter placed on the small beaker with the mercurous
nitrate solution, single crystals different in habit and
color (light-yellow prisms and very thin and fragile
colorless needles up to several mm in length) had
formed. Although it was not possible to separate larger
amounts of the so-obtained crystal mixtures for single-
phase measurements, XRPD analyses revealed a-
Hg2SeO3 (light-yellow prisms) and b-Hg2SeO3 (needles)
as the two phases in the bulk material. The presence of
the g-phase could only be detected by single-crystal
analyses of some of the colorless needles as described in
the following section.
2.2. Structure determination
In case of a-Hg2SeO3, hydrothermally grown crystals
were chosen for the data collection. Only very few
needle-shaped single crystals grown during the diffusion
experiment were suitable for subsequent structure
determinations. Their quality was checked by prelimin-
ary optical investigations under a polarizing microscope.
Intensity data for all crystals examined were measured
on a Siemens SMART system equipped with a CCD
camera and monochromatized MoKa radiation. Sur-
%
prisingly, for some of the needle shaped crystals, the cell
determination resulted in a completely different cell than
expected for b-Hg2SeO3, but for some crystals the C-
centered cell as determined by indexation of the powder
pattern was confirmed and in agreement with the
powder data. This was evidence that during the
diffusion experiment a third modification of Hg2SeO3
(or a completely different compound in the system Hg–
Se–O(–H)) had formed which had not been detected by
the previous XRPD analysis of the bulk material. The
2.3. Thermal analyses
The thermal behavior of single-phase material of a-
Hg2SeO3 and b-Hg2SeO3 was examined both in a closed
system using temperature-dependent Guinier photo-
graphs (sealed silicaꢁc1apillaries with 0.3 mm diameter,
heating rate of 8ꢀC h in the range 50–650ꢀC) and in an
1 For both polymorphs, the short c-axis is the needle axis.

38
M. Weil / Journal of Solid State Chemistry 172 (2003) 35–44
Table 1
Details of data collection and structure refinement
Compound
Diffractometer
a-Hg2SeO3
Siemens SMART CCD detector
b-Hg2SeO3
Siemens SMART CCD detector
g-Hg2SeO3
Siemens SMART CCD detector
(
MoKa; 0.71073
MoKa; 0.71073
MoKa; 0.71073
%
%
%
Radiation; l (A)
Temperature (ꢀC)
Crystal dimensions (mm3)
Crystal color; shape
Crystal system
22(2)
22(2)
22(2)
0.06 Â 0.04 Â 0.02
Light-yellow; pinacoid
Monoclinic
P21/n, 14
0.22 Â 0.007 Â 0.005
Colorless; needle
Orthorhombic
Cmc21, 36
0.22 Â 0.010 Â 0.009
Colorless; needle
Orthorhombic
rbam, 72
Space group, no.
Formula units Z
4
8.6177(8)
8
8
10.4293(16)
11.1528(3) / (11.142(3))a
(
a (A)
(
b (A)
(
c (A)
5.0224(4)
16.2291(4) / (16.265(4))a
5.0372(1) / (5.029(2))a
11.8522(19)
7.8324(12)
10.3958(9)
b (deg)
104.109(2)
436.37(7)
911.73(4) / 911.4(5)a
113.97
968.2(3)
121.03
3
V (A )
(
3
V/Z (A )
(
109.09
Formula weight (g ꢂ molꢁ1
)
528.14
78.472
8.039
528.14
75.116
7.695
528.14
70.738
7.247
m (mmꢁ1
)
X-ray density (g ꢂ cmꢁ3
)
Range ymin–ymax
2.76 – 30.23
ꢁ11 - 12
ꢁ6 - 7
2.22 – 24.99
ꢁ13 - 13
ꢁ19 - 19
ꢁ5 - 5
2.60 – 30.25
ꢁ14 - 13
ꢁ16 - 14
ꢁ10 - 11
SHELXTL [19]
SHELX97 [17]
3484
Range
h
k
l
ꢁ14 - 14
HABITUS [18]
SHELX97 [17]
4555
Absorption correction
Structure refinement
Measured reflections
SHELXTL [19]
SHELX97 [17]
5780
Independent reflections
Obs. reflections [I 42s(I)]
Ri
1279
1054
832
672
760
574
0.0464
0.0821; 0.2464
56
0.1021
0.1834; 0.6896
35
0.0484
0.2282; 0.5799
32
Trans. coef. Tmin; Tmax
Number of parameters
Ext. coef. (SHELXL97)
Diff. elec. dens. max; min
0.00466(16)
1.37 (0.79, Hg2);
ꢁ1.14 (1.31, Hg1)
0.0230
0.0468
1.046
0.00020(6)
6.75 (0.73, Se);
ꢁ5.64 (0.78, O1)
0.0879
0.2058
1.187
0.00027(2)
1.39 (0.84, Hg);
ꢁ1.34 (0.69, Hg)
0.0215
0.0489
0.970
ꢁ3
( (
A ) (dist. (A), atom)
(eꢁ
R[F2 4 2s(F2)]
wR2(F2 all)
Goodness-of-fit
CSD number
412302
412303
412304
a
(
Lattice parameters from the powder refinement with Si powder (a = 5.431021 A [16]), as internal standard, values from the single-crystal
measurement are included in parentheses.
open system under flowing N2 atmosphere using a
Mettler-Toledo DSC-25 system for DSC measurements
(heating rate 5ꢀC minꢁ1 in the range 35–450ꢀC, Al
crucibles) and a TG-50 system (heating rate 5ꢀC minꢁ1
in the range 35–750ꢀC, corundum crucibles) for
thermogravimetrical measurements. b-Hg2SeO3 was
employed as polycrystalline material as obtained after
the precipitation reaction; coarse-crystalline a-Hg2SeO3
as obtained after hydrothermal treatment was used for
TG/DSC, whereas the crystals were ground for tem-
perature-dependent Guinier measurements.
and standard deviations), the structure is considered as
essentially correct. Both single-crystal and powder
refinements did not reveal any reduction of the
symmetry and the results of the Rietveld refinement
[15] (Fig. 1) are in good agreement with the given single
crystal data. From the crystal chemical point of view,
the finally obtained model is plausible with reasonable
distances and angles for both SeO23ꢁ and Hg22þ groups as
discussed below.
All three modifications crystallize in new structure
types and are based on Hg22þ dumbbells and SeO23ꢁ
anions as the main building units. The linkage into the
three-dimensional network is achieved via common
oxygen atoms. Characteristic features of all structures
are open channels extending parallel to the shortest
crystallographic axis (Fig. 2). In case of a-Hg2SeO3, the
channels are running parallel to the b-axis and are made
3. Results and discussion
Although the refinement of b-Hg2SeO3 from the
single-crystal data is not very satisfactory (high R values

M. Weil / Journal of Solid State Chemistry 172 (2003) 35–44
39
Table 2
2
Ã
Ã
à Ã
(
Atomic coordinates and equivalent isotropic displacement parameters Ueq=(1/3)SiSjUij ai aj ꢂ ai aj ðA Þ
Atom
Wyckoff position
x
y
z
Ueq
a-Hg2SeO3
Hg1
Hg2
Se
4e
4e
4e
4e
4e
4e
0.83972(4)
0.88645(4)
0.03383(9)
0.8683(6)
0.0277(6)
0.1811(6)
0.26278(6)
0.20772(6)
0.74486(13)
0.6524(11)
0.0811(10)
0.6555(11)
0.40815(3)
0.65734(3)
0.87540(7)
0.7627(6)
0.8555(5)
0.7986(5)
0.02242(10)
0.02381(11)
0.01620(16)
0.0269(13)
0.0200(11)
0.0233(12)
O1
O2
O3
b-Hg2SeO3
Hg1
Hg2
Se
8b
8b
8b
8b
8b
8b
0.1133(2)
0.1130(2)
0.3231(6)
0.190(3)
0.306(3)
0.316(4)
0.24272(13)
0.0404(9)
0.0393(13)
0.0798(14)
0.003(9)
0.0311(9)
0.0331(9)
0.0306(17)
0.018(5)a
0.018(5)a
0.018(5)a
ꢁ0.08341(14)
0.1051(4)
0.071(2)
O1
O2
0.205(2)
0.101(3)
0.007(9)
0.412(8)
O3
g-Hg2SeO3
Hg
Se
16k
8j
0.33035(2)
0.03515(8)
0.4600(6)
0.1373(4)
0.10656(2)
0.19879(8)
0.1749(6)
0.2168(4)
0.25385(4)
0
0.02521(11)
0.0172(2)
O1
O2
8j
16k
0
0.1678(6)
0.0248(15)
0.0257(11)
a Restrained
Table 3
2
Anisotropic displacement parameters (A )
(
Atom
U11
U22
U33
U23
U13
U12
a-Hg2SeO3
Hg1
Hg2
Se
0.02401(17)
0.02780(19)
0.0197(4)
0.021(3)
0.02775(19)
0.02693(19)
0.0156(4)
0.025(3)
0.01518(15)
ꢁ0.00030(11)
0.00418(11)
0.00324(12)
0.0070(3)
0.004(3)
0.00049(13)
0.00213(14)
0.0016(3)
0.001(2)
0.01563(15)
0.0147(3)
0.033(3)
0.00037(12)
0.0011(3)
ꢁ0.012(3)
ꢁ0.001(2)
0.008(2)
O1
O2
0.021(3)
0.021(3)
0.016(3)
0.033(3)
0.018(3)
0.018(3)
ꢁ0.004(2)
0.009(2)
ꢁ0.003(2)
0.012(3)
O3
b-Hg2SeO3
Hg1
Hg2
0.0134(12)
0.0191(13)
0.0305(13)
0.0355(13)
0.0493(16)
0.0447(15)
ꢁ0.004(3)
ꢁ0.002(2)
0.0209(15)
0.0259(15)
ꢁ0.0010(10)
0.0019(11)
g-Hg2SeO3
Hg
Se
0.02833(17)
0.0159(4)
0.026(3)
0.01545(14)
0.0149(4)
0.027(4)
0.03186(17)
0.0207(4)
0.021(3)
0.00086(15)
0
0.00112(11)
0
0.00112(10)
ꢁ0.0006(3)
ꢁ0.017(3)
0.008(2)
O1
O2
0
0
0.027(2)
0.024(3)
0.027(3)
ꢁ0.001(2)
ꢁ0.009(2)
up of six-membered rings built of four mercury atoms
and two selenite(IV) groups. In case of b- and g-
Hg2SeO3, the size of the channels is larger due to
formation of eight-membered rings (four Hg atoms, four
SeO3 groups), and the direction of the channels is
parallel to the c-axis for both modifications. The
formation of six-membered rings in the a-modification
and eight-membered rings in the two others corresponds

40
M. Weil / Journal of Solid State Chemistry 172 (2003) 35–44
Table 4
(
Coordination around Hg and Se atoms with interatomic distances (A) and angles (deg), as well as bond valence sums (BVS) (v.u.).
a
a-Hg2SeO3
i
ii
ii
Hg1
O3
O3
2.152(5)
O3
3.056(6)
O2
Hg2
i
3.314(7)
3.288(8)
2.726(5)
98.03(10)
4.683(5)
4.141(5)
3.975(5)
2.5379(5)
ii
O3
O2
79.08(14)
84.64(17)
173.71(16)
2.612(5)
75.98(16)
107.06(12)
ii
Hg2
iii
iv
v
Hg2
O2
2.215(5)
O1
O1
O1
Hg1
iii
O2
O1
O1
O1
3.225(8)
2.509(5)
78.71(14)
130.6(2)
109.72(14)
3.352(7)
3.205(7)
2.545(5)
69.82(16)
114.22(12)
2.609(7)
5.0224(4)
3.205(7)
3.014(6)
117.54(10)
4.605(5)
4.127(5)
4.268(6)
4.754(6)
2.5379(5)
85.87(18)
89.27(19)
57.44(16)
153.51(14)
iv
v
Hg1
vi
vii
Se
O1
O2
O3
vi
O1
O2
O3
1.675(5)
101.2(3)
101.7(3)
2.609(7)
1.701(5)
102.0(3)
2.629(7)
2.655(8)
1.715(5)
vii
BVS
Hg1: 1.02
Hg2:1.06
Se:4.09
O1: 1.84
O2: 1.85
O3: 1.91
b,c
b-Hg2SeO3
i
ii
ii
iii
Hg1
O2
O2
O2
3.17(4)
2.66(4)
O3
3.46(6)
O1
O2
Hg1
2.24(4)
80.2(11)
87.6(13)
56.9(12)
73.7(13)
163.5(9)
2.53(5)
4.42(6)
5.34(5)
2.92(4)
97.3(11)
107.0(7)
3.17(4)
5.037(1)
2.65(6)
4.44(6)
2.96(4)
107.7(7)
4.72(4)
4.24(4)
4.22(4)
4.39(4)
4.44(4)
2.527(5)
ii
O2
O3
3.44(6)
2.73(5)
141.8(12)
55.2(12)
106.8(10)
iii
79.3(12)
105.3(12)
127.3(13)
109.8(8)
O1
O2
iii
iii
Hg1
iv
v
iv
iii
Hg2
O3
O3
2.37(5)
O1
3.41(5)
2.50(5)
O1
O1
Hg2
iv
3.16(6)
3.39(5)
2.65(4)
77.8(13)
108.8(8)
2.54(6)
5.037(1)
3.41(5)
2.84(5)
107.5(9)
4.83(5)
4.11(4)
4.20(4)
4.33(4)
2.520(5)
v
O1
O1
86.3(13)
78.3(15)
57.5(12)
162.7(10)
82.9(12)
141.3(15)
110.1(9)
iv
O1
Hg2
iii
Se
O1
O3
O2
O1
O3
O2
1.63(4)
100(2)
100.1(19)
2.53(5)
1.67(4)
104(2)
2.54(5)
2.65(6)
1.68(4)
BVS
Hg1:1.04
Hg2:1.02
Se:4.52
O1:2.08
O2:2.02
O3:1.80
d
g-Hg2SeO3
i
ii
iii
Hg
O2
O2
O1
O1
Hg
O2
2.208(5)
O2
O1
O1
Hg
i
2.794(9)
2.493(4)
92.93(19)
128.02(16)
121.16(11)
3.074(6)
3.646(7)
2.537(4)
91.66(9)
107.47(15)
2.607(7)
4.966(7)
4.004(3)
3.026(5)
106.42(12)
4.681(5)
4.372(5)
4.082(5)
4.457(6)
2.5267(7)
72.64(18)
80.44(18)
57.23(17)
162.69(11)
ii
iii
iv)
v)
Se
O1
O2
O2
O1
O2
O2
iv
1.690(6)
100.3(2)
100.3(2)
2.607(7)
1.705(4)
100.8(3)
2.607(7)
2.628(9)
1.705(4)
v
BVS
Hg:1.09
Se:4.05
O1:1.82
O2:1.93
a
1
2
1
2
Symmetry transformations used to generate equivalent atoms: (i) ꢁx+1, ꢁy+1, ꢁz+1; (ii) x+12; ꢁy+21; zꢁ ; (iii) x+1, y, z; (iv) ꢁx+23; yꢁ ,
ꢁz+32; (v) x, yꢁ1, z; (vi) xꢁ1, y, z; (vii) x, y+1, z.
b
Symmetry transformations used to generate equivalent atoms: (i) 2ꢁx+12; ꢁy+21; z+21; (ii) ꢁx+12; ꢁy+21; zꢁ ; (iii) 1ꢁx, y, z; (iv) x, ꢁy, zꢁ ; (v) x,
1
1
2
2
ꢁy, z+12:
c Distances and angles were calculated using the lattice parameters from the cell refinement of the powder data
d Symmetry transformations used to generate equivalent atoms: (i) 1ꢁx+12; ꢁy+12; ꢁz+12; (ii) x+0, ꢁy+0, ꢁz+21; (iii) ꢁx+1, y+0, ꢁz+21; (iv)
1
xꢁ ; ꢁy+12; z; (v) x, y, ꢁz.
2

M. Weil / Journal of Solid State Chemistry 172 (2003) 35–44
41
α
z
x
β
x
y
γ
x
y
Fig. 2. Projection of the crystal structures parallel to the b-axis (a-Hg2SeO3), and parallel to the c-axis (b-Hg2SeO3 and g-Hg2SeO3), respectively. The
mercury atoms are plotted as gray spheres and the selenite(IV) groups are represented as polyhedra.

42
M. Weil / Journal of Solid State Chemistry 172 (2003) 35–44
with the reaction conditions. While the densest a-
modification is formed hydrothermally under compara-
tively high pressure, b- and g-modification show a lower
density and were grown during a diffusion process under
normal laboratory conditions.
α
O1v
The Hg–Hg distances of the Hg22þ dumbbells (a:
(
(
(
2.5379(5) A; b: 2.527(5) and 2.520(5) A; g: 2.5267(7) A)
%
differ only slightly from the average distance d(Hg–Hg)
(
= 2.514 A calculated as the arithmetic mean of nearly
60 different Hg22þ dumbbells observed for various
mercury(I) oxo compounds. The dumbbells in b- and
g-Hg2SeO3 consist of crystallographically equivalent Hg
atoms, and a symmetrical oxygen environment around
the dumbbells is observed, whereas in a-Hg2SeO3 the
dumbbell is built of two inequivalent Hg atoms which
results in an asymmetrical oxygen coordination sphere
around these atoms (Fig. 3). Besides the neighboring
mercury atom, Hg atoms are bonded to three (Hg1,
O1
O1
β
O2
O2
Hg1 Hg1i
O2ii
O2
O2
O2 iii
(
d(Hg1–O) = 2.497 A) and four (Hg2, 2.571 A) oxygen
(
%
O3iii
(
O3
atoms in a-Hg2SeO3, five (Hg1, 2.70 A) and four (Hg2,
(
2.59 A) oxygen atoms in b-Hg2SeO3, and four oxygen
(
%
O1v
atoms in the g-modification (d(Hg–O)=2.566 A). Nor-
mally, interactions between an oxygen atom and a
mercury atom are considered as bonding, if the distance
O1
O1
(
d(Hg–O) is o3 A, which has empirically been derived
Hg2i
O3iv
Hg2
from the van der Waals radii of the Hg and O atoms
[23]. Some of the Hg–O distances are very close to this
O3
(
value (d(Hg2–O1) = 3.014(6) A in a-Hg2SeO3, and
O1
O1iv
(
d(Hg–O1) = 3.026(5) A in g-Hg2SeO3), and bond-
valence units for the distances are not negligible
(Table 4);2 this leads to the conclusion that these oxygen
atoms also belong to the first coordination sphere. More
remote oxygen atoms are located at distances d(Hg–O)
O1
O1
γ
O1
Hg
(
4 3.3 A. For all modifications, the angles +(Hg–Hg–O)
O2
O2
between the closely bonded oxygen atoms and the
mercury dumbbells are virtually linear ranging from
153.5ꢀ to 173.7ꢀ which is in accordance with the data for
+(Hg–Hg–O) angles observed for other mercury(I) oxo
compounds [25]. The Hg-O distances of these linear
bonded oxygen atoms are considerably shorter than the
Hg–O distances of the remaining O atoms.
O2
Hgii
O2i
O1iii
O1
The selenite(IV) groups display the well-known
trigonal pyramidal geometry with a similar distribution
Fig. 3. ORTEP plot [26] of the oxygen coordination sphere around the
mercury dumbbells. The ellipsoids are drawn at the 90% probability
level. For Symmetry operators, refer Table 4.
%
of bond length d(Se–O) and mean angles +(O–Se–O),
ꢀ
(
respectively (a-Hg2SeO3: 1.697 A, 101.6 ; b-Hg2SeO3:
ꢀ
ꢀ
(
(
1.66 A, 101.4 ; g-Hg2SeO3: 1.700 A, 100.5 ). Like in
other metal selenites(IV) (10), the lone-pairs of the SeIV
ions are stereochemically active and protrude into the
cavities of the structure. As well, the arrangement of the
SeO3 pyramids with the apices adjusted to each other
within the cavities is a frequently observed structural
unit in the crystal chemistry of oxoselenites(IV).
Upon heating in a closed system, b-Hg2SeO3 trans-
forms between 320(10)ꢀC and 340(10)ꢀC into the more
dense a-Hg2SeO3 (Eq. (1)), which in a disproportion
reaction transforms above 370(10)ꢀC into elementary
2 Bond-valence sums are in agreement with the expected values and
were calculated with the parameters provided by Brese and O’Keeffe
[24].

M. Weil / Journal of Solid State Chemistry 172 (2003) 35–44
43
0
100
200
300
400
500
600
700
10
8
exo
endo
6
4
2
0
0
100
200
300
Temperature [˚C]
Fig. 4. Thermoanalytical measurements of a-Hg2SeO3 and b-Hg2SeO3 (dotted lines).
400
500
600
700
mercury and b-HgSeO3 [9] as the only phase detected by
XRPD (Eq. (2a)). Above 480(10)ꢀC the capillaries
bursted due to the high internal pressure. Therefore no
other phases could be identified at higher temperatures
with this technique.
second step between ca. 400 and 500ꢀC, b-HgSeO3
converts in a redox-reaction (Eq. (3)) under loss of Hg
and SeO2 into the basic salt Hg3SeO6 [7] which is the
only phase detected by XRPD of the material heated up
to a temperature of T = 500ꢀC. The third step between
500ꢀC and 600ꢀC is assigned to the complete decom-
position of this phase (Eq. (4)). The observed mass
losses of ca. 38% after the first, and 67% after the
second step, are in agreement with the theoretical mass
losses of 37.98% and 63.23%, respectively.
Upon heating in an open system, both a- and b-
Hg2SeO3 decompose in a well-separated three-step
mechanism (Fig. 4). No phase transformation between
both Hg2SeO3 modifications was observed under these
conditions; b-Hg2SeO3 and a-Hg2SeO3 are stable up to
ca. 250ꢀC and 290ꢀC, respectively. In the first step, both
phases disproportionate into elementary mercury and a-
HgSeO3 (Eq. (2b)) which was identified by XRPD
analysis of the material heated to T = 360ꢀC. The
decomposition ranges for a-Hg2SeO3 (ca. 290–350ꢀC)
and b-Hg2SeO3 (ca. 250-330ꢀC) slightly differ due to the
different particle sizes of the employed material. In a
comparison measurement with ground crystals of a-
Hg2SeO3, the decomposition range was considerably
lowered and similar to that of the polycrystalline b-
Hg2SeO3 material. At ca. 380ꢀC (b-Hg2SeO3 batch) and
415ꢀC (a-Hg2SeO3 batch) a-HgSeO3 transforms into b-
HgSeO3 (Eq. (2c)), which is accompanied by a strong
and narrow endothermal effect and was confirmed by
subsequent phase analyses. The obtained phase trans-
formation temperature for the measurement of poly-
crystalline b-Hg2SeO3 is in very good agreement with
the value of 376.6ꢀC for the a-b transformation of
polycrystalline a-HgSeO3 given in Ref. [27].3 In the
b-Hg2SeO3;s-a-Hg2SeO3;s ðclosed systemÞ;
a-; b-Hg2SeO3;s-b-HgSeO3;sþHg;g ðclosed systemÞ;
ð2aÞ
a-; b-Hg2SeO3;s-a-HgSeO3;sþHg;gðmÞ ðopen systemÞ;
ð2bÞ
ð2cÞ
ð1Þ
a-HgSeO3;s-b-HgSeO3;s ðopen systemÞ;
4b-HgSeO3;s-Hg3SeO6;sþ3SeO2;gðmÞ þ Hg;gðmÞ ðopen systemÞ;
ð3Þ
Hg3SeO6;s-SeO2;gðmÞ þ 2O2;gðmÞ þ 3Hg;gðmÞ ðopen systemÞ:
ð4Þ
Acknowledgments
Ithank G. Koch (University of Gie ꢀen, Germany) for
the temperature-dependent Guinier photographs, and
F. Werner (TU Wien, Austria) for the thermal analysis
3 Interestingly, no a-b transformation has been observed by DSC
techniques on heating coarse-crystalline material of a-HgSeO3 under
similar conditions [9].

44
M. Weil / Journal of Solid State Chemistry 172 (2003) 35–44
measurements using the Mettler system. Dr. E. Halwax
(TU Wien, Austria) kindly provided valuable comments
on the manuscript.
[15] A.C. Larson, R.B. Von Dreele, General Structure Analysis
System (GSAS), LANSCE, MS-H805, Los Alamos National
Laboratory, Los Alamos, NM, USA, 2000.
[16] P.J. Mohr, B.N. Taylor, CODATA, Recommended Values of the
Fundamental Physical Constants, National Institute of Standards
and Technology, Gaithersburg, MD, USA, 1998.
[17] G.M. Sheldrick, SHELX97, Program for Crystal Structure
Solution and Refinement, University of Go.ttingen, Germany,
1997.
References
[1] Gmelins Handbuch der anorganischen Chemie, 34 Quecksilber
B4, 8th Edition, Verlag Chemie, Weinheim, 1969, p. 1094.
[2] M. Koskenlinna, J. Valkonen, Acta Crystallogr. C 51 (1995)
1040.
[18] W. Herrendorf, HABITUS, Program for Optimization of the
Crystal Shape for the Numerical Absorption Correction, Uni-
versities of Karlsruhe and Gieꢀen, Germany, 1993/1997.
[19] Bruker AXS, SHELXTL, Version 5.1, Madison, WI, USA, 1998.
[20] H.D. Flack, Acta Crystallogr. A 39 (1983) 876.
[3] M. Koskenlinna, J. Valkonen, Acta Crystallogr. C 52 (1996) 491.
[4] M. Koskenlinna, J. Valkonen, Acta Crystallogr. C 52 (1996) 1070.
(
[5] C. Stalhandske, Acta Crystallogr. B 34 (1978) 1049.
[21] A.K. Spek, PLATON, a multipurpose crystallographic tool,
Utrecht University, Utrecht, The Netherlands, 2001.
[6] E. Dorm, Acta Chem. Scand. 23 (1969) 1607.
[7] M. Weil, Z. Naturforsch. 57b (2002) 1043.
[22] E. Dowty, ATOMS for Windows, Version 5.1, Shape Software,
521 Hidden Valley Road, Kingsport, TN USA, (2000).
!
[23] D. Grdenic, Q. Rev. Chem. Soc. 19 (1965) 303.
[8] M. Weil, U. Kolitsch, Acta Crystallogr. C 58 (2002) i47.
[9] M. Weil, Solid State Sci. 4 (2002) 1153.
[24] N.E. Brese, M. O’ Keeffe, Acta Crystallogr. B 47 (1991) 192.
[25] N.V. Pervukhina, S.A. Margarill, S.V. Borisov, G.V.
Romanenko, N.A. Pal’chik, Russ. Chem. Rev. 68 (1999) 615.
[26] G.G. Gospodinov, B.G. Bogdanov, Thermochim. Acta 71 (1983)
391.
[10] V.P. Verma, Thermochim. Acta 327 (1999) 63.
[11] J.J. Berzelius, Afh. Fys. Kemi Mineral 6 (1818) 42.
[12] J.J. Berzelius, Ann. Chim. Phys. 9 (1818) 160, 225, 337.
[13] J.J. Berzelius, Schweiggers J. Chem. Physik. 23 (1818) 309, 430.
[14] P.-E. Werner, L. Eriksson, M. Westdahl, J. Appl. Crystallogr. 18
(1985) 367.
[27] C.K. Johnson, ORTEPII., Report ORNL-5138, Oak Ridge
National Laboratory, TN, USA, 1976.